1. Field of the Invention
This invention relates to fluid to fluid heat exchangers and particularly to structural features thereof reducing the likelihood of fluid mixing as may result from leakage caused by wear, corrosion, imperfect braze joints or the like.
2. Description of the Prior Art
In a common form of fluid to fluid heat exchanger, plate elements and spacer elements are effectively stacked or assembled so that they define multiple superimposing or side by side fluid flow passages. Through use of applied manifolds, or other means, first and second fluids are directed through alternating passages, with a transfer of heat taking place from the fluid of higher temperature to the fluid of lower temperature through the plate separating adjacent passages. Usually a strip of corrugated fin material is placed in each flow passage to make the heat transfer process more efficient and to promote structural strength and rigidity. After assembly, and in a preferred joining process, parts are united into a one-piece structure by a brazing process in which plate elements are made fast to spacer elements upon which they superimpose and the peaks and valleys of the corrugated fin material attach to overlying and underlying plate elements. In various ways, braze material is introduced at joints where parts contact one another, and, in the presence of heat and pressure, the material flows to fill the defined joints. The braze connection is multi-functional. It connects the parts in a uniform assembly. It provides for heat flow with minimal thermal resistance, and, it seals the defined joints against fluid leaks. Since at least one of the flowing fluids is often under relatively high pressure, great care is usually taken to insure that the braze joints are sound and well sealed.
Heat exchangers as described have enjoyed a long commercial success. They lend themselves particularly well to fabrication using light weight metals. They are easy to manufacture, all of the braze joints being effected in a single operation, and they are highly efficient.
However, use requirements are sometimes quite severe. In some applications, for example, even minor amounts of leakage from one flow circuit to another cannot be tolerated. When made under carefully controlled conditions and subjected to repeated inspections, a leak proof heat exchanger can be produced. Such manufacture may not always be economically feasible, however, and is in any event no guarantee that leaks will not develop in service. Common causes of leaks are weakly brazed joints not adequately resistant to vibration and pressure stresses, and imperfections in a plate element developing pin hole leaks in the manufacturing process or as a result of corrosion. One or both of the fluids put through the heat exchanger may be corrosive or entrain corrosive materials. One practiced brazing process is carried out in a salt bath and, with occasionally imperfect results, subsequent flushing procedures are used to wash salt residue from interior passages.
In addition to efforts made to make the heat exchanger leak proof, concurrent efforts have been made to render leaks harmless, that is, to prevent fluid interchange from one flow circuit to another. In one prior art example, shown in P. Bathla U.S. Pat. No. 3,825,061, issued July 23, 1974, plate elements are configured as unitary tubes leaving the fluid conducted therethrough no opportunity to leak through brazed joints. Also, through a use of superstructure built up at either end of the heat exchanger, drained collection chambers are provided for fluid which may leak past those braze joints which at other locations separate the fluid circuits. The prior art construction adequately serves its intended purpose. Certain disadvantages and limitations attend its use, however. Thus, the integrally formed tubes and components comprising the end superstructure are not usual or conventional in plate and fin constructions. Their use adds substantially to the difficulties and cost of manufacture. Further, the prior art device does not take into account or provide for leakage through plate elements defining the flow passages. As seen above, these can be the source of leaks as well as can the braze joints.